132 SUMMARY OF CURRENT RESEARCHES RELATING TO 



to the number of the spines. After many generations of selection the 

 family becomes much diversified with respect to the characters selected, 

 so that the coefficient of correlation between parent and offspring may 

 become almost as high as in populations. 



In respect to the three characters referred to in the preceding 

 paragraph, parents that deviate from the general mean produce progeny 

 that deviate from the mean in the same direction as the parents, but to 

 a less extent. Regression of the progeny towards (but not to) the general 

 mean occurs. 



The variations which are inherited m later generations are some- 

 times considerable in extent, so that they may be characterized as 

 saltations (or mutations, if these be defined as marked inherited varia- 

 tions). But most of the inherited variations are very slight. After 

 many generations of descent from a single progenitor a family will show 

 differentiation into many hereditarily diverse stocks, differing hereditarily 

 as to particular characters and also as to combinations of characters. In 

 general, this interesting investigation shows that in Dijh/gia corona a 

 population consists of many hereditarily diverse stocks ; and that a 

 single stock, derived by fission from a single progenitor, gradually 

 differentiates into such hereditarily diverse stocks ; so that by selection 

 marked results are produced. 



Feeding of Amceba.*— S. 0. Mast and E. M. Root have made a 

 series of observations on the feeding of Amoeba with special reference 

 to the surface-tension theory. The Amoeba observed was a large lobose 

 form corresponding, at least superficially, to the ordinary descriptions of 

 Amccha proteus. They found tliat the Amoeba feed at times almost 

 exclusively on Rotifers, at other times largely on Faramoccium. They 

 capture the Rotifers by flowing around the foot of the point of attach- 

 ment to the sul:)stratum. The Rotifer responds by contracting and 

 forcing the Amoeba back, after which it extends again, and the Amoeba 

 begins to flow out over it. In the meantime the foot begins to digest, 

 and gradually the Rotifer weakens. They continue thus sometimes for 

 days before the Rotifer is swallowed. When the Amoeba are feeding 

 on Paramcecium they assume a sort of mushroom shape with a serrate 

 edge consisting of numerous short pseudopodia. The Infusorians tend 

 to come to rest between and under these pseudopodia by which they are 

 usually surrounded, but sometimes the ends of the pseudopodia approach 

 each other before they are fully extended, and cut the Paramcccium in 

 two. To cut a Paramwcium in two with a fine glass fibre requires a 

 pressure of approximately 9 mms. If the pseudopodia have the same 

 cutting quality as the glass fibre, and if their movement is clue to a 

 change o:' surface-tension, it requires, to perform the work involved, a 

 reduction in surface-tension of at least lll.s dynes per centimetre at the 

 tips of the pseudopods. If the ends of the pseudopods fuse so as to 

 take on tne form of a ring round the Parama'cium, and if the cutting 

 is due to constriction in this ring, and if the constriction is due to a 

 change in surface-tension, the work involved requires a minimum reduc- 

 tion along the inner surface of the ring of 383 dynes per centimetre. 



* Journ. Esper. ZooL, xxi. (1916) pp. 31-50 (5 figs.). 



